The present invention is particularly applicable to thermit welding and will be described with particular reference thereto; however, it will be apparent that the invention has broader applications for a variety of retention and reaction vessels and molds where thermal stability and high refractory properties are required for the reaction and retention of molten metals. Thermit reaction welding is extensively used to provide on-site interconnection between current carrying and/or load bearing members. For example, the weld produced in thermit reaction welding can be used to splice cables to one another, to interconnect cables to ground rods or bus bars, to structurally and electrically interconnect bus bars, to connect cable to pipes, rails, lugs as well as many other applications where current carrying, strength and durability properties are required. In this connection, the resulting weld from the thermit reaction has current carrying capacity and strength properties equalling or exceeding those of the connector members being joined. The thermit weld connection provides a permanent molecular bond between the connector members that does not loosen or corrode, can be made with inexpensive light weight, on-site equipment without external power or heat, and without specialized job skills. The welded joint can be visually checked for quality. The thermit reaction welding process is well suited for joining cables together or to ground or support structures of the same or similar materials and, therefore, the present invention will be particularly described with reference thereto. At the same time however, it will be appreciated that the invention is applicable to the thermit welding of a wide variety of components and structures whereby it will be understood that the description with reference to the welding of ground cables is intended to be illustrative of the invention and not limiting with respect thereto.
The basic thermit reaction takes place within the confines of a high temperature reaction mold. The mold includes a weld cavity having a shape generally defining the intended configuration of the weld and in connection with the welding of cables, passages are provided for inserting and supporting the terminal portions of the cable in prepositioned relation to one another within the weld cavity. The weld cavity communicates with an upwardly opening crucible by means of a tap hole. The tap hole is sealed by a disc and the crucible is filled with a combination of powdered metals, one of which is an oxide of the weld metal and the other which is generally aluminum powder. A suitable starting material such as magnesium powder is sprinkled over the top of the powdered metals. By means of a flint ingitor or other starting device, the starting material is ignited which initiates an exothermic reduction reaction in the crucible between the powdered metals. This reduction reaction produces a molten base metal and an aluminum oxide slag. The molten base metal melts through the disc and flows downwardly into the weld cavity and around the cables, thereby locally melting the ends of the cable and, upon solidification, forming an integral connector joint conforming to the weld cavity contour.
These reactions molds are assembled from multiple parts to provide the necessary separation of the parts for release of the welded joint following the thermit welding process. The number of parts and their interfitting varies from application to application as required to provide for the separation without destruction of the mold parts in order that reaction mold may be reused on a semi-permanent basis. Because the reactions molds must withstand extremely high temperatures, in the range of 5000.degree. F. or higher, graphite has been generally employed as the reaction mold material. Graphite provides the advantages of high temperature resistance and nonwetting of the slag or the weld material, the latter allowing for easy removal of the slag and the completed welded joint from the mold. However, graphite has significant thermal conductivity which requires that special equipment such as gloves or the like be provided for the welder to permit safe separation of the mold parts. Moreover, graphite is an expensive material and, because of its physical characteristics, does not lend to the forming of mold parts other than by conventional machining operations such as cutting, milling, boring and the like to provide the necessary cavities and passage ways in the assembled mold. Consequently, a graphite mold is expensive, and is complex and difficult to manufacture and use.
It has been suggested that some of the above-mentioned difficulties associated with the manufacture of graphite molds could be overcome by the use of moldable refractory materials to form the mold parts. In this respect, the mold parts would be formed to the desired shape relative to the parting surfaces required for mold separation of the parts to accommodate removal of the molded joint. Therefore, the parts would have the required cavities and connector openings appropriately formed during the molding process, advantageously overcoming the need for numerous and time consuming machining operations. Such approaches are focused on forming the mold parts from an aggregate of a suitable mixture of temperature resistant and refractory materials which are adhesively bonded by organic resins. However, these previous efforts have not provided satisfactory results in the thermit reaction welding process. More particularly, the materials heretofore proposed as binders have included curable water-based organic binders of high moisture content. Therefore, unless the formed mold parts are thoroughly dried over extended drying periods, and then assembled and used rather quickly, the mold may have an unacceptable moisture content at time of use. This retention of moisture is potentially dangerous and undesirable in that, when a thermit reaction is initiated, the resulting temperatures in the mold may be sufficient to produce fragmentation or fracturing of the mold components due to water-steam expansion therein. Further, these binders tend to be hygroscopic, whereby the potential for absorbing moisture prior to use or during periods of storage or nonuse increases potential for fracturing and fragmentation. Further, either as a result of the moisture problem and/or the aggregates used, the aggregate often contaminates the base metal through impurities which can enter the molten metal. This can result in inclusions which impair the electrical and mechanical properties of the end product. Because the thermit weld relies in its simplicity upon a visual indication of acceptable weld quality, this potential for contamination can lead to a rejection of such a mold material when the user requires stringent weld quality. Moreover, such inclusions of aggregate in the end product necessarily results in mold errosion and a lessening of useful mold life.